1. Field of the Invention
The present invention relates to a process for producing chlorosulfonyl isocyanate. More specifically, the present invention relates to a process for producing chlorosulfonyl isocyanate with high purity in a high yield.
2. Description of the Prior Art
Chlorosulfonyl isocyanate, also called N-carbonylsulfamoyl chloride, has a chemical formula of O.dbd.C.dbd.NSO.sub.2 Cl and is a useful compound attracting, particularly in recent years, much attention as intermediate raw materials for the production of medicines, agricultural chemicals, sweetners and the like and improving agents for synthetic fibers, synthetic resins and like materials.
It has been known for long that chlorosulfonyl isocyanate is obtained by reacting sulfur trioxide with cyanogen chloride. See for example Org. Synth. Coll. 5, 226-231; Angew. Chem. Internat. Edit. 7, No. 3, 172-173 (1968); Chem. Ber. 89, 1071-1079, (1956), German Pat. 928,896 (1955) and British Pat. 774,276 (1957).
The former two of the above literature disclose a process for producing chlorosulfonyl isocyanate which comprises adding sulfur trioxide to liquid cyanogen chloride at a low temperature of -5.degree. C. or below. This process, however, has problems in that it must use as large an amount of liquid cyanogen chloride as 1.5-3 times that of sulfur trioxide, thereby being not suited for commercial production both economically and from the viewpoint of safety, that the yield is as low as about 60 to 62%, and that the obtained product is not of satisfactory qualities to meet the demands of market.
The latter three disclose a process which comprises mixing sulfur trioxide with cyanogen chloride at a temperature above 100.degree. C., e.g. 100.degree.-200.degree. C. This process also has problems of difficulty in providing the product with satisfactory qualities, since the flow rates of sulfur trioxide and cyanogen chloride are difficult to control and there form large amounts of byproducts depending on the reaction conditions employed.
On the other hand, U.S. Pat. No. 3,375,088 discloses a process for obtaining a high-purity chlorosulfonyl isocyanate, which comprises treating the crude chlorosulfonyl isocyanate obtained by reacting sulfur trioxide with cyanogen chloride with benzonitrile or its derivatives. However, in employing this process it is necessary to separately provide the process of reacting sulfur trioxide with cyanogen chloride and that of purifying the crude chlorosulfonyl isocyanate, and to further provide a process for recovering and purifying the treating agent including benzonitrile. Accordingly, the whole process is complexed, thereby requiring high equipment cost and time-consuming operation, and hence this process has not been fully satisfactory for commercial production.
In view of the foregoing, we have proposed, for the purpose of solving the problems encountered in the above processes, a process for producing chlorosulfonyl isocyanate which comprises adding cyanogen chloride to liquid sulfur trioxide and reacting them while maintaining the temperature of the reaction system at 20.degree. to 50.degree. C. (Japanese Patent Application Laid-open No. 77855/1988). This process has made it possible to commercially produce in a simple manner chlorosulfonyl isocyanate with a relatively high purity in a relatively high yield, and to solve to some extent the problems of conventional processes, i.e. low yield and purity resulting from formation of byproducts. However, with ever enhancing demands from the market, there has been desired development of a process which can provide chlorosulfonyl isocyanate with a still higher purity in a still higher yield.